1. Field of the Invention
The invention is generally related to scanning microscopes, such as tunneling or atomic force microscopes. More particularly, this invention relates to probes and methods of mounting probes onto holder chips used in these microscopes.
2. Description of the Related Art
Atomic force microscopy (AFM) is extensively used in microelectronics as an electrical characterisation tool in order to determine the carrier distribution inside semiconductor devices. Electrically conductive probe tips are needed for these measurements and therefore different probe concepts have been developed. Such probe tips are integrated into cantilever beams and may be divided into two classes: coated Si tips, and pyramidal metal or diamond tips. In xe2x80x9cThe fabrication of a full metal AFM probe and its applications for Si and InP device analysisxe2x80x9d, T. Hantschel et Al., Proc. Micromachining and Microfabrication conference, 1999 a method for manufacturing full metal probes, applying the so-called moulding technique, is disclosed. Prior to the mounting of the probe into a scanning microscope, the probe needs to be fixed to small holder chips with millimeter dimensions (e.g. 3 mm long, 1.5 mm wide and 0.3 mm thick). In case of Si probes, i.e., a probe tip with a silicon cantilever, these holder chips may be formed out of the same wafer from which the Si tips and cantilevers are manufactured. This is however not possible for pyramidal probe tips made by the so-called moulding technique, where the holder chip has to be attached in a separate step to one longitudinal end of the cantilever. The electrical probes, comprising a cantilever beam with integrated pyramidal metal or diamond tips, have to be mounted to small holder chips before they may be used in electrical atomic force microscopy (AFM).
Three solutions have been developed so far for the mounting of moulded AFM probes. In one method a glass wafer is anodically bonded to the Si wafer and the holders are then diced by sawing. This method requires for example high temperature processing, thorough cleaning of the glass surface, separate voltage source. This mounting procedure is well known for the fabrication of Si3N4 probes as disclosed by T. R. Albrecht et Al. in, xe2x80x9cMicrofabrication of cantilever styli for the atomic force microscopexe2x80x9d, J. Vac. Sci. Technol. 8(4), pp. 3386-3396, 1990. In a second method holder chips are formed by locally electroplating the Si wafer. Full metal probes with integrated holders have been manufactured in this way as disclosed by J. P. Rasmussen et Al. in xe2x80x9cFabrication of an all-metal atomic force microscope probexe2x80x9d, Proc. Of Transducers ""97, pp. 463-466, 1997. Several layers of resist have to be spun on the substrate in order to form a mould having the desired thickness to form in this mould the holder chip by electroplating. In a third method the holder chips are glued to thin contact areas which are formed during cantilever etching. C. Mihalcea et Al. illustrates this application in xe2x80x9cFabrication of monolithic diamond probes for scanning probe microscopy applicationsxe2x80x9d, Appl. Phys., A66, pp. S87-S90, 1998. Due to its simplicity gluing is today mostly used for the mounting of electrical AFM probes made by the so-called moulding technique. FIG. 1 illustrates the mounting of the probe, comprising the contact area (7), the probe tip (2) and cantilever (1) to a Si holder chip (5). In case of a non-conductive glue (3) a silver paint dot (4) bridges the step between contact area (7) and chip (5). An additional metal coating (6) on the tip side is needed in order to obtain a good electrical connection between tip and chip.
Moulded AFM probes, e.g., full metal probes or Si cantilevers with integrated metal and diamond tips according to the prior art, were mounted to holder chips by gluing. This procedure has three main disadvantages:
The non-conductive glue requires an additional thin metal coating (6) on the tip side. This additional metal coating increases the tip radius slightly leading to a smaller resolution of the scanned image.
The fast-drying glue allows only separate probe mounting as the solder chips have to be mounted one at a time. This method is thus not very suitable for mass fabrication of probes for electrical scanning microscopes.
The thus mounted probe suffers from a low mechanical stability. The probe may e.g., often be used only in contact mode AFM because sharp resonance peaks as required in tapping mode will normally not be obtained with glued probes. In tapping mode the probe is in intermittent contact with the sample and oscillates at a frequency close to its resonance frequency. This method will reduce the wear of the probe tip and extend the lifetime of the probe. The gluing connection was often not rigid enough for tapping mode application. The poor resonance behavior of the glued probes might be caused by two factors: a high loss of energy due to the use of flexible glue, and a free-standing probe which is fixed only in the middle to the holder chip. More rigid glue might reduce the energy losses but other factors like viscosity and drying time of the glue need also to be matched. FIG. 2 shows a typical resonance curve obtained with test glued probes. Multiple unsharp or dull peaks were typically observed instead of one sharp peak (FIG. 2a). As shown after magnification, the main peak is very broad and therefore not suited for tapping mode measurements (FIG. 2b).
Other disadvantages of the glue technique include the difficulty of fixing the probe over the whole overlap region of contact area (7) and the holder chip (5) and the removal of the excess glue after attaching the holder chip. The excess glue is often found back on the cantilever after mounting.
An aim of the invention is to provide a method of mounting holder chips to probes, used in scanning microscopes, providing a direct electrical connection between the cantilever with probe tip and the holder chip. This method does not require additional processing after mounting of the holder chip to establish such electrical
A further aim of the invention is to establish a connection between the holder chip and the probe such that during the step of forming said connection the original or as-deposited position of the holder chip with respect to the probe remains essentially the same.
A further aim of the invention is to provide a method to mount holder chips to probes without substantially changing the tip geometry and conducting properties of tip and cantilever.
A further aim of the invention is to obtain a rigid probe-holder chip connection having good mechanical properties fit for e.g., tapping mode AFM. The connection between holder chip and contact area is established substantially over the overlap region of both parts.
An aim of the invention is to provide a method of mounting holder chips to probes, used in scanning microscopes, providing a direct electrical connection between the cantilever with probe tip and the holder chip. This method does not require additional processing after mounting of the holder chip to establish such electrical connection.
Another aim of the invention is to obtain probes with a soldered connection between holder chip and cantilever, having the holder chip essentially at its original position with respect to the cantilever. Holder chip and cantilever are positioned along the same longitudinal axis.
In one embodiment, the invention provides a method for mounting a holder chip to a probe. This method comprises the steps of:
applying a solderable material on the holder chip;
applying a solder paste to the contact area;
mounting the holder chip to the contact area on top of the solder paste; and
soldering the holder chip to the probe.
In a first embodiment the solderable material deposited on the holder chip and the contact area is disclosed. Preferably the same solderable material is deposited on both parts. The solderable material may be a stack of Ti:W, Au and may further comprise Ni.
In a second embodiment the solder paste is disclosed. This solder paste may comprise a flux. The solder paste should fix the holder chip after depositing the holder chip to the paste, such that the position of the holder chip relative to the contact area remains essentially unchanged during the further soldering of the holder chip. The solder paste may be based on IPA or water-soluble fluxes. The solder paste may also be fluxless.
In a third embodiment the step of establishing the solder connection between the holder chip and the contact area is disclosed. Preferably the assembly of holder chip on solder paste on contact area is heated on a hotplate. During the heating step a solder connection between holder chip and contact area is established The solder will wet substantially the whole overlap region but will not flow outside this region.